meta and para substitution effects on the electronic state energies and ring‐expansion reactivities of phenylnitrenes

Abstract

ABSTRACT:The electronic structures of the triplet ground states and first three excited singlet states for phenylnitrene, 14 meta-, and 17 para-substituted congeners have been characterized using density functional theory and multireference second-order perturbation theory (CASPT2). Ring expansion pathways to form didehydroazepines have activation enthalpies of about 9 kcal · mol −1 and are fairly insensitive to substitution-in the case of the strongest para donor, MeNH-, this barrier increases to about 13 kcal · mol −1 . The trends in state energies as a function of substitution are rationalized using a (2,2) configuration interaction theory and qualitative molecular orbital theory. Analysis of spin-orbit coupling in the nitrenes using the same model in conjunction with explicit calculation of spin-orbit coupling matrix elements rationalizes why electron donating substituents increase rates of intersystem crossing.

2-Pyridylnitrene and 3-Pyridazylcarbene and Their Relationship via Ring-Expansion, Ring-Opening, Ring-Contraction, and Fragmentation

“…It is possible to make reasonable estimates for open-shell singlet species at the B3LYP/6-31G* level with use of the Cramer-Ziegler sum method. 24 The latest value for the energy gap between the triplet and the open-shell singlet (OSS) phenylnitrene is ca. 15 kcal/mol, 25 whereas a value of ca.…”

“…23 We calculate the T 0 -S 1 gap ( 3 A 00 -1 A 00 ) in 19 as 18.6 (19.7) kcal/ mol at the CASSCF (CASPT2) levels, and 16.2 kcal/mol at the B3LYP/6-31G* level using the Cramer-Ziegler correction. 24 Thus, were it to occur, the overall carbene-nitrene rearrangement reaction, 10 ( 1 A 0 ) f 19 ( 1 A 00 ) would be exothermic by about 39-48 kcal/mol. The calculated activation energy for N 2 loss from diazo compound 9 is 41 kcal/mol (B3LYP/6-31G*).…”

Communication: Energetics of reaction pathways for reactions of ethenol with the hydroxyl radical: The importance of internal hydrogen bonding at the transition state

“…͑Representative applications may be consulted for details. [24][25][26][27][28][29][30][31][32] ͒ Recently, calculations on the reactions of OH with propene were reported by Izsák et al, 33 who employed CASPT2 geometry optimizations. Harding et al 34,35 have obtained good results with CASPT2-optimized transition states.…”

Nitrene-Carbene-Carbene Rearrangement. Photolysis and Thermolysis of Tetrazolo[5,1-a]phthalazine with Formation of 1-Phthalazinylnitrene, o-Cyanophenylcarbene, and Phenylcyanocarbene

“…Energies of open shell species were calculated by the sum method. 19 Geometries, energies, and imaginary frequencies are listed in the Supporting Information.…”